LIFE-CYCLE AND MODE OF INFESTATION OF MYZOSTOMA-CIRRIFERUM (ANNELIDA), A SYMBIOTIC MYZOSTOMID OF THE COMATULID CRINOID ANTEDON-BIFIDA (ECHINODERMATA)

Eight different stages succeed one another in the life cycle of the my zostomid Myzostoma cirriferum, viz. the embryonic stage, 4 larval stag es, and 3 postmetamorphic stages. Fertilization is internal. Embryogen esis starts after egg laying and takes place in the water column. Cili ated protrochophores and trochophores are free-swimming. Ciliated meta trochophores (i.e., 3 d old larvae) bear 8 long denticulate setae and form the infesting stage. They infest the host Antedon bifida through the feeding system of the latter: they are treated by hosts as food pa rticles and are caught by the host's podia. By means of their setae, m etatrochophores attach on the host's podia and are driven by the latte r in the pinnule groove where they eventually attach and undergo metam orphosis. Juveniles and early males remain in the pinnules. They attac h to the ambulacral groove through parapodial hooks and produce locali zed pinnular deformations. Late male and hermaphroditic individuals mo ve freely on their host. They occur outside the ambulacral grooves and are located respectively on the pinnules, the arms or the upper part of the calyx of the host, depending on their stage and size. The succe ss of the Myzostoma cirriferum-Antedon bifida symbiosis is ensured by the usually high density of the hosts' populations, the way the myzost omids reproduce (reproduction occurs year-round) and their effective m ode of infestation.